Plasticizers are incorporated into a resin (usually a plastic or elastomer) to increase the flexibility, workability, or distensibility of the resin. The largest use of plasticizers is in the production of “plasticized” or flexible polyvinyl chloride (PVC) products. Typical uses of plasticized PVC include films, sheets, tubing, coated fabrics, wire and cable insulation and jacketing, toys, flooring materials such as vinyl sheet flooring or vinyl floor tiles, adhesives, sealants, inks, and medical products such as blood bags and tubing, and the like.
Other polymer systems that use small amounts of plasticizers include polyvinyl butyral, acrylic polymers, nylon, polyolefins, polyurethanes, and certain fluoroplastics. Plasticizers can also be used with rubber (although often these materials fall under the definition of extenders for rubber rather than plasticizers). A listing of the major plasticizers and their compatibilities with different polymer systems is provided in “Plasticizers,” A. D. Godwin, in Applied Polymer Science 21st Century, edited by C. D. Craver and C. E. Carraher, Elsevier (2000); pp. 157-175.
The most important chemical class of plasticizers is phthalic acid esters, which accounted for about 84% worldwide of PVC plasticizer usage in 2009. However, there is an effort to decrease the use of phthalate esters as plasticizers in PVC, particularly in end uses where the product contacts food, such as bottle cap liners and sealants, medical and food films, or for medical examination gloves, blood bags, and IV delivery systems, flexible tubing, or for toys, and the like. As a result, there is a need for non-phthalate, mono- or diester plasticizers, particularly oxo-ester plasticizers, that can be made from low cost feeds and employ few manufacturing steps in order to have comparable economics with their phthalate counterparts.
To this end, suggested substitutes for phthalates recently have included biphenylester-based plasticizers. For instance, U.S. Patent Publication No. 2014/0275609 teaches, among other things, the manufacture of dimethylbiphenyl compounds containing significant amounts of the 3,3′-dimethyl, the 3,4′-dimethyl and the 4,4′-dimethyl isomers. Such compounds can be economically produced by hydroalkylation of toluene and/or xylene followed by dehydrogenation of the resulting (methylcyclohexyl)toluene and/or (dimethylcyclohexyl)xylene product. As also taught in U.S. Patent Publication No. 2014/0275609, the resultant mixture can be used as a precursor in the production of biphenylester-based plasticizers by, for example, oxidizing the methyl-substituted biphenyl compounds to convert at least one of the methyl groups to a carboxylic acid group and then esterifying the carboxylic acid group with an alcohol, such as an oxo alcohol.
The proposed synthesis of dimethylbiphenyl compounds includes a first step of hydroalkylation of an aromatic hydrocarbon (e.g., benzene, toluene, and/or xylene, among others), which produces desired phenylcyclohexane and/or alkylated phenylcyclohexane intermediates, as well as undesired cyclohexane and/or alkylcyclohexane intermediate byproducts. These intermediates and intermediate byproducts can be dehydrogenated to form, respectively, (i) the desired biphenyl and/or dialkylbiphenyl product (e.g., dimethylbiphenyl) and (ii) benzene and/or alkylated benzene (e.g., toluene). Commercially desirable efficiency is achieved by separating and recycling the benzene and/or alkylated benzene for utilization as additional hydroalkylation feed.
The present inventors have found that the efficiency of the process can surprisingly be further improved by an additional, separate dehydrogenation of the cyclohexane and/or alkylcyclohexane intermediate byproducts in the absence of C12 or greater hydrocarbons to provide benzene and/or alkylated benzene for the additional hydroalkylation feed.
Additional references of interest may include U.S. Pat. Nos. 6,730,625 and 6,037,513; U.S. Patent Publication Nos. 2014/0275605, 2014/0275606, 2014/0275607, 2014/0323782; Sinfelt, J. Mol. Cat. A., 163 (2000), at 123; and Sinfelt et al., J. Phys. Chem., 64 (1960), at 1559.